Numerous types of touch sensor or key have already been proposed.
In particular, as described in EP-A-0 079 711, GB-2 138 571, U.S. Pat. No. 4,224,615, and in an article by M. J. Bradshaw et al entitled "New form of liquid crystal touch switch," published in Electronic Letters, Vol. 8, No. 25/26, December 1982, pp. 1106-1108, proposals have been made for keys each comprising a liquid crystal cell which exhibits a change in capacitance on being subjected to a mechanical force, and which is connected in a detection bridge circuit in order to detect any such change in capacitance.
However, the devices described in the above-mentioned documents do not make it possible simultaneously to make use of the liquid crystal cells to provide a conventional display, at any rate not by using means which are cheap and simple.
The object of the present invention is to provide a touch sensor which makes use of the specific property of liquid crystals, in particular of helical nematic liquid crystals, whereby a mechanical force exerted on a helical nematic liquid crystal in the excited state gives rise to a temporary change in the disposition of its molecules and this gives rise to a considerable, but short-lasting change in its dielectric constant, while nevertheless enabling the liquid crystal cells to be used simultaneously in conventional manner for display purposes.